Method and apparatus for setting up a communication with a target base station in a cellular or cordless mobile telecommunications system

ABSTRACT

Methods, systems and network elements are described to improve the speed of handover in a mobile radio telecommunications system. In one embodiment a mobile terminal ( 114 ) transmits a list of addresses of network nodes ( 102, 103, 105, 122, 125 ) to a target base station ( 106 ) in preparation for setting up a new radio link to the target base station ( 106 ). The list of addresses can be used by the target base station ( 106 ) to select a node ( 122, 125 ) which is common to both the old and the new communication path and to address this node directly without requiring a cross-over switch network query. The new path ( 122, 110, 106 ) can then be set up quickly ready for the handover to target base station ( 106 ). In a further embodiment the network distributes pre-authenticated signatures for a mobile terminal ( 114 ) to the nodes ( 102, 103, 105, 122, 125 ) of the network currently supporting a communication. This allows local verification of a mobile terminal ( 114 ) when it attempts to set up a new radio link with a target base station ( 106 ). In yet a further embodiment the target base station ( 106 ) starts fast power control with the mobile terminal ( 114 ) before the new communications path ( 125, 110, 106 ) has been set up through the network.

The present invention relates to a method of forward handover (sometimesknown as “hand off”) or for certain types of call restablishment in acellular or cordless mobile telecommunications system. Embodiments ofthe present invention may find application in many forms of mobiletelecommunications system independent of the access method, e.g. FDMA,TDMA, FDMA/TDMA, CDMA, FDMA/TDMA/CDMA, TDD, CDMA/TDD or similar methods.The present invention may also relate to a mobile telecommunicationsnetwork and a method of operating the same.

TECHNICAL BACKGROUND

Cellular mobile telephone systems rely on the reuse of radio frequenciesin different cells or radio coverage areas. When a mobile terminal movesfrom communication with a base station in one cell (original basestation) to another it is necessary to “handover” the mobile terminal tothe base station (target base station) of the new cell. Handovers may bepassive or active, i.e. a handover may be necessary when there is noactive communication and the mobile terminal is “camped” on the currentcell and must be transferred to the new cell (passive handover) or whenthere is an active communication which must be transferred from thecurrent to the target base station (active handover). In order to avoidloss of data during an active handover it is preferred if the handoveris quick. In order to improve the speed of handovers both “seamless” and“soft” handovers have been proposed. A seamless handover is a “hard”handover (new connection to the target base station is made at the timeof loss of the old connection) in which the network connections up tothe target base station are reserved and synchronised so that when theactual handover takes place there is the minimum of interruption betweenbreaking the communication with the old base station and connectingthrough to the new base station. In a soft handover the mobile terminalcommunicates with both the old base station and the target base stationat the same time and the network may decide, based on certaincommunication qualities or other criteria, when the signals arriving viathe target base station are acceptable and the link to the original basestation may be broken.

Hard handover methods may suffer from suddenly varying signal strength.The deterioration of the signal quality of communications with theoriginal base station may be masked by shadowing and widely varyingfading on the current communication channel so that the reduction insignal quality which triggers the handover may occur suddenly andcatastrophically (for example, the “corner effect”). In this situationcommunication with the original base station is lost suddenly and ahandover procedure which relies on a handover request being sent to theoriginal base station may fail. Schemes have been proposed to deal withthis situation, e.g. on loss of communication with the original basestation the mobile terminal attempts a forward handover, i.e. it triesto make contact with a target base station and the network attempts toset up the new route through the network from the target base station.Setting up the new network path takes some time, particularly as thenetwork has not been prepared in any way for this emergency operation,so that it is almost impossible with this method not to loose some data.Very often the complete communication is lost.

An alternative is to allow communication with more than one base stationat the same time so that sudden loss of communication with one basestation has no affect on the current communication. A method and asystem for providing a communication with the mobile terminal throughmore than one base station during the handover process are disclosed inU.S. Pat. No. 5,625,876. Using this system, a communication between themobile terminal and the end user is not interrupted by a handoff fromthe original base station to a target base station. This type ofhandover may be considered as a “soft” handover in that communicationwith the target base station is established before communication withthe original base station is terminated. When the mobile terminal is incommunication with two base stations, a single signal for the end usermay be created from the signals from each base station by a cellular orpersonal communication system controller.

In systems in which a mobile terminal may communicate with several basestations at the same time, e.g. CDMA systems, mobile terminal assistedhandoff may operate based on the signal strength of beacon or pilotsignals from several sets of base stations as measured by the mobileterminal. An Active Set is the set of base stations through which activecommunication is established. The Neighbour Set is a set of basestations surrounding an active base station comprising base stationsthat have a high probability of having a pilot or beacon signal strengthof sufficient level to support communication of adequate quality. TheCandidate Set is a set of base stations having a pilot or beacon signalstrength of sufficient level to establish communication.

When communications are initially established, a mobile terminalcommunicates through a first base station and the Active Set containsonly the first base station. The mobile terminal monitors the pilot orbeacon signal strength of the base stations surrounding at and each ofthese is allocated to the Active Set, the Candidate Set, or theNeighbour Set. When a pilot or beacon signal of a base station in theNeighbour Set exceeds a predetermined threshold level, the base stationis added to the Candidate Set and removed from the Neighbour Set of themobile terminal. The mobile terminal communicates a message to theoriginal base station identifying the new base station. A cellular orpersonal communication system controller decides whether to establishcommunication between the new base station and the mobile terminal.Should the cellular or personal communication system controller decideto do so, the cellular or personal communication system controller sendsa message to the new base station with identifying information about themobile terminal and a command to establish communications therewith. Amessage is also transmitted to the mobile terminal through the originalbase station. The message identifies a new Active Set that includes theoriginal and the new base stations. The mobile terminal searches for thenew base station transmitted information signal and communication isestablished with the new base station without termination ofcommunication through the original base station. This process cancontinue with additional base stations.

When the mobile terminal is communicating through multiple basestations, it continues to monitor the signal strength of the basestations of the Active Set, the Candidate Set, and the Neighbour Set.Should the signal strength corresponding to a base station of the ActiveSet drop below a predetermined period of time, the mobile terminalgenerates and transmits a message to report the event. The cellular orpersonal communication system controller receives this message throughat least one of the base stations with which the mobile terminal iscommunicating. The cellular or personal communication system controllermay decide to terminate communications through the base station having aweak pilot or beacon signal strength.

The cellular or personal communication system controller upon decidingto terminate communications through a base station generates a messageidentifying a new Active Set of base stations. The base station throughwhich communication is established sends a message to the mobileterminal. The cellular or personal communication system controller alsocommunicates information to the relevant base station to terminatecommunications with the mobile terminal. The mobile terminalcommunications are thus routed only through base stations identified inthe new Active Set. In a cellular or personal communication telephonesystem, maximising the capacity of the system in terms of the number ofsimultaneous telephone calls that can be handled is also extremelyimportant. If the mobile terminal is communicating simultaneously withseveral base stations, for example, when the mobile terminal is close tothe boundaries of several different cells, additional radio and networkresources are occupied which reduces the total capacity of the system.Hence, it is advantageous to limit the number of simultaneouscommunications of the same data using up radio capacity. Further, a highbit-rate, high priority mobile terminal which approaches a cell boundarymay be operating at high power due to poor communication channels. Thishigh power signal may seriously increase the noise level in the targetcell. Generally, fast power control of the mobile terminal from thetarget base station will only be effective after the soft handover iscomplete. During this delay the users in the target cell may receivepoor communication quality and/or loss of their calls.

As mobile cellular telephone systems have become more sophisticated, thehandover requirements have also become more complex. In particular,there is an increase in the types of communications supported by mobiletelecommunication systems and the requirements of these communicationsmay be widely different. Important parameters of any communication maybe its urgency, i.e. how quickly must the handover take place and howmuch delay or interruption is acceptable, its bandwidth, its minimum orpreferred signal quality, its data rate, its priority, i.e. is thecommunication a premium service which may take priority over lessimportant messages. Conventionally, certain types of data are consideredto require a very quick handover with little loss or delay of thecommunication, e.g. telephone conversations. The increasing use ofInternet telephones indicates, however, that delays in voicecommunications may be acceptable provided there is a significant costsaving. In addition, there is an increasing use of voicemail. Sometelephone users only use their fixed telephones for outgoing calls andswitch all ingoing calls to voicemail leaving them undisturbed untilthey wish to create a communication. With voicemail it is only necessarythat the voice information is recorded accurately, exactly when itarrives is not of such a great importance. These changes in voicecommunications mean that different levels of service may be provided bythe mobile communication network operator, e.g. a high price voicecommunication with high urgency and priority and a low price voicecommunication in which delays in duplex communications are possible andhave to be accepted. On the other hand, certain types of datatransmissions suffer greatly if there is an interruption duringhandover. Also, the bandwidth for different types of voice or datatransmissions may vary widely, e.g. video, computer file downloads,voice or fax. Thus, there are several different qualities of service(QoS) which may be required for any particular transmission. During ahandover the required QoS of the current transmission may be one of theparameters which influences the handover decision, i.e. can the new pathsupport the required QoS. Checking the QoS for the new path requiresknowledge about the properties of the new route from the target basestation through the network to the end user to decide whether or not itmeets the minimum QoS or the present QoS of the current transmission.This increase in sophistication of the handover decision increases itscomplexity and the time taken for its successful completion. Forexample, if one of the QoS requirements of an existing transmission isits priority i.e. it has a higher priority than other transmissions inthe old cell or new cell it may be necessary to terminate lower prioritycommunications in the target cell or to arrange for them to be handedover to another cell before the high priority communication can itselfbe handed over.

Summarising the above, there is a need for a very fast handover whichcan accommodate QoS requirements such as priority, urgency, bandwidth orcell or packet delay and which is easy to implement.

SUMMARY OF THE PRESENT INVENTION

The present invention includes a method of operating atelecommunications system in which mobile terminals may communicate withbase stations over an air interface and a communications network isprovided for linking each base station to other points in the networkvia one or more nodes, a communication to another user terminal beingsupported by one or more radio links between one or more current basestations and a mobile terminal through a plurality of current nodes ofthe system, the method comprising the steps of: providing to the mobileterminal information defining explicitly a least some of the currentnodes of the communications network supporting the communication; and,in preparation for setting up a further radio link between the mobileterminal and a target base station: the mobile terminal transmitting theinformation defining explicitly a least some of the current nodes of thecommunications network supporting the communication to the target basestation. The method also includes setting up a new path from one of thenodes explicitly defined in the information to the target base stationthus reusing part of the old communication path. The explicitinformation may include the addresses of the nodes or aliases of theaddresses, for instance.

The present invention also includes a telecommunication system in whichmobile terminals communicate with base stations over radio links,comprising: a communications network for linking each base station toother points in the network via one or more nodes, a communicationbetween a mobile terminal and another user terminal being connected viaone or more radio links to one or more current base stations and througha plurality of current nodes of the network, wherein, in preparation forsetting up a further radio link between the mobile terminal and a targetbase station, the mobile terminal is adapted to transmit to the targetbase station information defining explicitly at least some of thecurrent nodes of the communications network supporting thecommunication. The explicit information may include the addresses of thenodes or aliases of the addresses, for instance.

The present invention includes a method of operating atelecommunications system in which mobile terminals may communicate withbase stations over an air interface and a communications network isprovided for linking each base station to other points in the networkvia one or more nodes, a communication to another user terminal beingsupported by one or more radio links between one or more current basestations and a mobile terminal through a plurality of current nodes ofthe system, the method comprising the steps of: providing the mobileterminal with pre-authenticated reference data for that mobile terminal;copying the pre-authenticated reference data to at least some of thecurrent nodes of the communications network supporting thecommunication; and, in preparation for setting up a further radio linkbetween the mobile terminal and a target base station: the mobileterminal transmitting at least a part of the pre-authenticated referencedata to the target base station.

The present invention also includes a telecommunication system in whichmobile terminals communicate with base stations over radio links,comprising:

a communications network for linking each base station to other pointsin the network via one or more nodes, a communication between a mobileterminal and another user terminal being connected via one or more radiolinks to one or more current base stations and through a plurality ofcurrent nodes of the network, wherein, the system is adapted to providethe mobile terminal with pre-authenticated reference data for thatmobile terminal and for copying the pre-authenticated reference data toat least some of the current nodes of the communications networksupporting the communication, and, in preparation for setting up afurther radio link between the mobile terminal and a target basestation, the mobile terminal is adapted to transmit to the target basestation at least a part of the pre-authenticated reference data.

The present invention includes a method of operating atelecommunications system in which mobile terminals may communicate withbase stations over an air interface and a communications network isprovided for linking each base station to other points in the networkvia one or more nodes, a communication to another user terminal beingsupported by one or more radio links between one or more current basestations and a mobile terminal through a plurality of current nodes ofthe system, the method comprising the steps of: a target base stationbeginning fast power control with the mobile terminal in preparation forsetting up a further radio link between the mobile terminal and thetarget base station before the path through the network supporting thefurther radio link is complete.

The present invention also includes a telecommunication system in whichmobile terminals communicate with base stations over radio links,comprising: a communications network for linking each base station toother points in the network via one or more nodes, a communicationbetween a mobile terminal and another user terminal being connected viaone or more radio links to one or more current base stations and througha plurality of current nodes of the network, wherein, in preparation forsetting up a further radio link between the mobile terminal and a targetbase station, the target base station is adapted to begin fast powercontrol with the mobile terminal station before the path through thenetwork supporting the further radio link is complete.

The present invention also includes a mobile terminal for use in atelecommunication system in which mobile terminals communicate with basestations over radio links, the mobile terminal being adapted to transmitto a target base station information defining explicitly at least someof the current nodes of the communications network supporting anexisting communication in preparation for setting up a further radiolink between the mobile terminal and the target base station.

The present invention also includes a network element for use in atelecommunication system in which mobile terminals communicate with basestations over radio links, the network element being adapted to receiveinformation explicitly defining at least some of the current nodes ofthe communications network supporting an existing communication, and forsetting up a communications path with a further network element from theexplicit information in preparation for setting up a further radio linkbetween the mobile terminal and the target base station.

The linking technical concept between the embodiments is that oflimiting the possibly disturbing effects of a mobile terminal close tothe border of one cell on a neighbouring cell by reducing the handovertime and/or by starting the fast power control as soon as possible.Handover time may be reduced by transmitting explicit informationdefining the nodes supporting the existing communication to the targetbase station from the mobile terminal and/or using pre-authenticatedsignatures in the handover request which allow verification of thesignature in a part of the network local to the target base station. Itis preferred in any of the systems, network elements or methodsaccording to the present invention that the explicit informationdefining nodes supporting the existing communication includes explicitinformation defining at least two levels of a hierarchicaltelecommunications network. The explicit references to the nodes of thecurrent communication allow rapid setting up of a partly new path to thetarget base station while maintaining optimal use of existing paths ofthe communication.

The information relating to the existing nodes supporting a call may beused as part of several procedures, e.g. a handover procedure, a callre-establishment, to determine whether a member of the Candidate set ofthe mobile terminal is added to the Active set, or whether a member ofthe Neighbour set of the mobile terminal is added to the Candidate set.

The present invention will now be described with reference to thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a telecommunications network inaccordance with a first embodiment of the present invention.

FIG. 2 is a schematic representation of a switch rerouter in accordancewith the present invention.

FIG. 3 is a schematic representation of part of the network of FIG. 1.

FIG. 4 are schematic look-up tables use to control a switch rerouter inaccordance with an embodiment of the present invention.

FIG. 5 is a schematic representation of a network in accordance with asecond embodiment of the present invention.

FIG. 6 is a schematic representation of part of the network of FIG. 5during a soft handover.

FIG. 7 is a representation of the network after the soft handover ofFIG. 6 is complete.

FIG. 8 is a schematic representation of a network supporting a softhandover in accordance with a third embodiment of the present invention.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The present invention will be described with a reference to certainembodiments and drawings but it is not limited thereto but only by theclaims. In particular, the present invention will mainly be describedwith reference to handover procedures, but the present invention mayalso find application in call re-establishment after loss of thecommunication channel between a mobile terminal and its current basestation or for determining which target base station should be includedwithin the Active set of the mobile terminal. Further, the presentinvention will mainly be described with reference to cellular mobiletelecommunication systems, but the present invention is not limitedthereto and may be applied to any mobile telecommunication system, e.g.a cordless telecommunication system such as DECT, which routes messagesthrough a network to transceiver base stations.

A first embodiment of the present invention will be described withreference to FIG. 1 which is a schematic representation of a cellularmobile telephone system 10. It may include a first network 20 whichprovides communication routing to a plurality of base stations 21 to 23,each of which has a radio coverage area or “cell”. Network 20 includesthe necessary control and execution units to be able to set up acommunication between any of the mobile terminals 33 to 34 with anyother mobile terminal or with any other user in fixed wireline or mobiletelephone networks 50, 60 such as existing public or private mobiletelephone networks, PSTN's, or commercial data networks. A portion ofthe network 10 may include a core network or backbone network 40 forrouting calls to and from networks 20, 50 and 60.

Let us assume that an existing call has been set up between a fixedterminal 62 associated with network 60 and a mobile terminal 33. Thecommunication path traverses network 60, network 10 and network 20whereby the switching node 31 is the gateway switch to network 20.Switches 26, 27 and 28 as well as base station 22 and the radio airinterface between base station 22 and mobile terminal 33 support thecommunication. If the mobile terminal 33 now moves into the radiocoverage area 37 of base station 23 it is necessary to handover thecommunication from base station 22 to base station 23. There are variouspossibilities for setting up the new path in network 20 from basestation 23 to gateway switch 31, whereby some of these routes may take alonger time to set up than others (involve more rerouting procedures atswitches) and some may only provide a limited quality of service, e.g.bandwidth, packet delay. To reduce the handover time to a minimum anoptimised new route in network 20 is preferably determined and resourcesallocated and synchronised when necessary before the handover iscompleted, whereby at least part of the old route is re-used whereverpossible. In accordance with the present invention at least a part ofthe series of switching nodes 31, 26, 27, 28 are preferably reused inthe new network connection from base station 23 to gateway switch 31.The more of the current route supporting the current communication tobase station 22 which can be re-used, the less time it is necessary toset up the communications through the new switching nodes present in thenew route to base station 23, however, such a new route may be longer orcause more traffic problems than another route. Hence, an optimum newroute may include a choice between different potentially conflictingrequirements and limitations. There are various known methods forselecting a new route in network 20 to support a handover from one cellto another. One technique involves sending a handover request frommobile terminal 33 to the original base station 22 providing anindication of the base station 23 which is to be the target base stationof the handover. The mobile terminal 33 may assist in determining whichof the base stations is the most suitable for handover by providing tonetwork 20 indications of the signal strength or signal quality fromsignals received by mobile terminal 33 from target base stations such as23. Alternatively, the mobile terminal 33 may contain sufficientintelligence to decide on the target base station based on its ownmeasurements. Any handover decision algorithm which involves the mobileterminal 33 sending a handover request to the original base station 22may suffer from the problem that the mobile terminal 33 losescommunication with the old base station. For instance, if the mobileterminal 33 is close to the point at which a handover is required, thesignal strength and quality of transmissions to and from base station 22is likely to be poor. In large metropolitan areas shadowing effects mayresult in a very sudden temporary loss of communication with originalbase station 22 with the risk of a total loss of communication. On theother hand, mobile terminal 33 is moving towards base station 23 so itis to be expected that the signal strength and quality from base station23 will continuously improve. In accordance with one embodiment of thepresent invention it is preferred if the mobile terminal 33 sends thehandover request to the target base station 23 rather than to theoriginal base station 22. Further, in order to speed up handover inaccordance with the present invention, mobile terminal 33 sendsinformation to base station 23 in the handover request which explicitlyidentifies at least some of the switching nodes 26 to 28 and 31 ofnetwork 20 which are involved in supporting the current communication.This information can be used by network 20 to determine an optimum newpart of the path required to the base station 23 from one of theswitching nodes 26, 27 or 28 while re-using the rest of the network. Inaddition the mobile terminal may send, in accordance with anotherembodiment of the present invention a pre-verified and authorisedsignature with the handover request. As the nodes of network 20 whichsupport the current communication have been informed of the signaturesuseable by mobile terminal 33, the time for verification andauthentication of the mobile terminal is reduced.

The switch which is the last switch in the old communication path whichcan be re-used is called the “cross-over switch”. In accordance with thepresent invention, the information in the handover request specifyingthe switching nodes involved in the current communication should be anexplicit reference to these nodes which are potential cross-overswitches rather than only a reference to virtual connections or linksused in the existing communication. For example, in accordance with thepresent invention a list of network addresses or address aliases of theswitching nodes 26, 27, 28 and 31 may be transmitted with the handoverrequest from mobile 33 to the target base station 23. The purpose ofsending the addresses to the target base station is to allow the networkto immediately identify the switching nodes involved in the currentcommunication and to ascertain whether any of the nodes 26, 27, 28 and31 lies on a potential new route through network 20 to base station 23.In particular, network 20 can determine from the transmitted list ofaddresses which of the nodes 26–28, 31 in the existing communicationpath could or should be the start of the new part of the path to basestation 23. The new part of the path to base station 23 must be set upfrom the crossover switch and therefore it is preferred in accordancewith this embodiment of the present invention if the nodes whoseaddresses are transmitted to the target base station 23 by the mobileterminal 33 are switches which are capable of setting up such a new paththrough network 20. The mobile terminal 33 does not need, in accordancewith the present invention, to transmit the addresses of all theswitching nodes involved in the current communication. For instance,only the addresses of switches which may serve as crossover switches maybe transmitted as not every switch may be able to reroute acommunication and thus be suitable as a potential cross-over switch.

As an example of the first embodiment of the present invention, aseamless hard handover in an ATM (asynchronous transfer mode) network 20will be described, however, the present invention is not limitedthereto. It is anticipated that the exact form of communicationtransfer, whether Internet protocol (IP), Mobile IP, ATM or any otherprotocol, is not a limitation on the present invention although it maywell affect the detailed implementation thereof.

In an ATM network a communication is broken up into discrete “packets”or “cells” whereby in the following the word packets will be used inorder to avoid confusion between these communication cells andgeographical cells. Each packet is sent one at a time through thenetwork 20. In order to make optimal use of each connection, packetsfrom many different callers may simultaneously share the samecommunication link in the network thus requiring that each packet isdistinguished from other packets in some way. ATM packets are dividedinto two sets or fields of information. One set is the information whichthe user intends to transmit and the other set is called the “header”.Additional fields may be added where more complex switching techniquesare used. The header contains routing information, including a virtualchannel identifier (VCI). The VCI is an indication which is assigned tothe packet which allows an ATM switch to know where to send the packetnext based on the switch port where the packet has been received. Aconventional ATM switch has several input and output ports. The ATMswitch keeps a record of the input port, the VCI for the input port, theoutput port and the VCI for the output port for example in a “look-uptable”. For every possible input port and incoming VCI combination,there is a corresponding output port and outgoing VCI combinationprogrammed into the look-up table. When an ATM switch receives a packetat the given port, the ATM switch will find the row in the look-up tablewhich has the incoming VCI and input port corresponding to the receivedpacket. The ATM switch will then switch or route the packet to theoutput port which appears in the same row and replaces the incoming VCIin the packet header with the outgoing VCI.

It is preferred in accordance with the present invention if the headerof the ATM packets used for communication to and from a mobile terminal21 to 23 includes an identifier for that mobile terminal, i.e. asignature or signatures. Methods of generating secure signatures areknown to the skilled person. In accordance with the present invention, anumber of secure signatures may be provided to a mobile terminal from asuitable authentication unit within the mobile telecommunications systemand this list of signatures may be communicated to all nodes of thenetwork 20 which supports a communication to or from the mobile terminal21–23. ATM switches in network 20 which are suitable as potentialcrossover switches will be called “switch rerouters” in accordance withthe present invention. These switch rerouters have the capability toreroute a packet, i.e. to provide the packet with a different VCI and tosend it to a different port than previous packets of the samecommunication. A switch rerouter 27 in accordance with one embodiment ofthe present invention is shown schematically in FIG. 2. The switchrerouter 27 performs the rerouting of the virtual channel connectionduring handover. It is an ATM switch having input ports 55 and outputports 56. At each input port 55 of the switch rerouter 27, there is amonitor/comparitor 57 which has read/write access to the switch'slook-up table 58. In the table 58 information may be stored which notonly associates a virtual channel connection with an incoming andoutgoing VCI as well as the incoming and outgoing switch ports of theconnection but also the signature or signatures of the mobile terminal33 using the connections may be stored optionally. FIG. 4 shows oneexample of an implementation of this embodiment which includes a look-uptable in an ATM switch rerouter in accordance with the present inventionbut the present invention is not limited to look-up tables as onlymethod of implementation of this embodiment. With reference to FIGS. 2to 4, when the mobile user 33 is communicating via the base station 22there is a virtual channel connection established through network 20from base station 22 to gateway switch 31. When the communication is setup, the signature or list of signatures of mobile terminal 33 iscommunicated to all nodes of the network supporting the call, e.g. basestation 22, switch 28, etc. by network 20 and the signature or list ofsignatures is written into the look-up table of each such node on therow including the virtual channel indication VCI of the communication(FIG. 4 a). Communications which leave the base station 22 are assigneda virtual channel identifier VC1 by base station 22. The packets aretransmitted via output ports 70 to input ports 66 of switch 28 (which isa normal ATM switch). Switch 28 reads the VCI from the header of eachpacket, consults its look-up table (FIG. 4 a) using VC1 and input port66 as search parameters and assigns a virtual channel identifier VC3 toeach packet to let ATM switch rerouter 27 know where the communicationis coming from and what the packet's final destination is. The packet istransmitted via output port 64 of switch 28 to input port 62 of the ATMswitch rerouter 27. The monitor/comparitor 57 of ATM switch rerouter 27reads VC3, consults its look-up table (FIG. 4B) and checks that the“Enabled” flag has been set to “1”, meaning that this is an activecommunication. The packet is then assigned the virtual channelidentifier VC4 to instruct the next switch 26 where the packet isdestined to go.

Packet communications travelling towards base station 22 from outsidenetwork 22 will arrive at ATM switch rerouter 27 from switch 26 at inputport 73 with a virtual channel identifier VC1. The monitor/comparitor 57at input port 73, consults its look-up table and assigns a virtualchannel identifier of VC2 to indicate to switch 28 that the packet isdestined for base station 22, and checks that the “Enabled” flag hasbeen set to “1” (FIG. 4B). The packet leaves via output 61 and entersinput port 63 of switch 28. Switch 28 reads VC2 and after consulting itslook-up table (FIG. 4A) gives the packet the virtual channel identifierVC0 which is switched to output port 65 and is transmitted to basestation 22 via input port 69.

In accordance with this embodiment of the present invention, a specificmessage sent between gate way switch 31 and base station 22 contains acode in its header which is read by monitor/comparitor 57 in each switchrerouter 27 and causes each switch rerouter 27 to include its ownnetwork address or its address alias into the message and to forward italong the established communications path to base station 22. Thus, thebase station 22 receives, via this message, a list of the addresses oraliases of all the switch rerouters along the communication path of thecurrent communication. Base station 22 then transmits the list ofaddresses of the current switch rerouters on the current path to mobileterminal 33 via the air interface. The mobile terminal 33 stores thisinformation. Note that switches along the route which are normal ATMswitches and not switch rerouters do not have a monitor comparitor 57and therefore do not react to the address interrogation signal and theiraddresses are not included in the list of addresses sent to the mobileterminal 33. Similarly, an address interrogation signal is transmittedfrom base station 22 towards gateway 31 as part of the call set up andthe list of addresses of switch rerouters along this path is stored ingateway 31.

In this embodiment of the present invention, call handover is initiatedby the mobile user 33. The handover procedure may be initiated by anyoneof several well known means. For example, the mobile terminal 33 maymonitor power from each of the base stations in its vicinity andinitiates a handover whenever the signal strength received from theterminals original base station approaches some lower threshold of QoSand there is at least one other base station from which an acceptablyhigh power level is detected. The mobile terminal 33 then transmits ahandover request to the selected target base station 23. The handoverrequest may include the or a current signature of mobile terminal 33,the list of addresses of the switch rerouters 26, 27 currentlysupporting the communication and optionally a definition of the minimumand/or current QoS of the existing communication, e.g. bandwidth,urgency, maximum packet delay, priority. Base station 23 is providedwith sufficient topological information relating to the network andprocessing power to decide if one or more of the switch rerouters 26, 27included in the list lie on the usual communication path for this basestation. Optionally, base station 23 may also have sufficientinformation over the topology of network 20 so that it can determine thelength and number of switching nodes between base station 23 and one ormore of the switch rerouters 26, 27 included in the transmitted list ofaddresses. Further, base station 23 is able to verify that the signatureof the mobile terminal allows access to the network. Preferably, basestation 23 does not have to carry out an interrogation request tocentralised network elements such as an authentication centre in orderto determine this. Instead, it is preferred for time saving reasons ifeither the current list of all signatures of mobile terminals camped onor communicating with the network is transmitted to each base station sothat the base station 23 can rapidly determine if the signature isenabling, or, which is the preferred method, the signature of the mobileterminal 33 is verified when the base station requests the relevantswitch rerouter to set up a new path. The base station 23 selects one ofthe switch rerouters 26, 27 as the crossover switch for the handoverdependent upon a suitable algorithm, for instance, the route with theminimum number of hops between base station 23 and the crossover switchor by any other suitable method. Having selected the appropriatecrossover switch, let us say switch rerouter 27, the base station 23sends a signal to switch rerouter 27 to set up a path between switch 27and base station 23, to reserve the necessary resources in this new pathand to synchronise ready for a transmission from mobile terminal 33 tobase station 23. This signal from the base station 23 includes thecurrent signature of mobile terminal 33, so that when the signal arrivesat crossover switch 27, the switch rerouter 27 can determine from itslook-up table whether the signature already exists in the look-up tablebecause it has been previously communicated to this switch-rerouter 27by the network. If it does the switch rerouter 27 may set up a new path.If not the switch 27 sends a message to the base station 23 “mobile notauthenticated” and the base station 23 either sends a “handover requestdenied” signal to the mobile terminal 33 or the base station 23 requestsauthentication from network 20 in the conventional way. Assuming theswitch rerouter 27 finds the signature innits look-up table, itdetermines from the look-up table which virtual connections and inputand output ports are involved in the communication to be re-routed. Thenswitch rerouter 27 sets up a communication path to base station 23parallel to the communication to base station 22. For instance, switchrerouter 27 sets up a communication to switch 24 so that packetsreceived at input port 73 are transmitted via output port 60 but given aVCI of VC7 which indicates to switch 24 that the communication is forbase station 23. As this new communication is not active at the presenttime, the “Enabled” column is set to “0”. Similarly, incoming packetsfrom base station 23 via switch 24 with a VCI of VC5 arriving at port 59are assigned a VCI of VC4 and transmitted via port 74 to indicate toswitch 26 that these packets are for gateway 31. Again the enabledcolumn is set to “0” in the look-up table as this is not an activecommunication yet. When this procedure is complete switch 26 informs thebase station 23 of the completion and the base station 23 acknowledgesthe handover request to mobile terminal 33.

When mobile terminal 33 receives the handover request acknowledgement itsends a handover command to base station 23. Base station 23 transmitsthe handover command to crossover switch 27 using the VCI now associatedwith this connection. When crossover switch 27 receives the handovercommand it acknowledges the command to the mobile terminal 33 via basestation 23 and switches the existing communication from the currentoutput ports to the new output ports. It does this by changing theenabled connections to not enabled and the not enabled connections toenabled. That is the entries in the look up table (FIG. 4B) are changedfrom “1” to “0” and vice versa. At the same time mobile terminal 33begins to communicate via base station 23 and sends a control messagethat the handover is completed. The handover complete message is sent toswitch 27 which then instructs switch 28 in the old communication pathto tear down the communication. Network 20 may then update the signatureor list of signatures for mobile terminal 33 in each of the nodes of thenew communication path including the mobile terminal 33 so that the oldsignature used for setting up this new communication is deleted andcannot be used by an interloper who has received the signature sent inclear by mobile 33 with the handover request.

If the target base station 23 does not recognise any of addresses of theswitch rerouters on the current path transmitted by the mobile terminal,the handover request may be transferred to the next higher switch and soon until a switch is reached which has one of the named switch reroutersin the transmitted list within its normal communication pattern.Alternatively, if the target base station 23 does not recognise any ofaddresses of the switch rerouters on the current path transmitted by themobile, a completely new path establishment can be initiated.

In accordance with a modification of this embodiment the mobile terminal33 may optionally communicate with at least 2 or more base stations atthe same time. This may be carried out, for example, in an FDMA/TDMAsystem such as the European GSM mobile telecommunications network by aseparate receiver at each base station which is specifically allocatedto search for and synchronise with handover requests from mobileterminals not within the radio cell of its own base station.Alternatively, techniques such as disclosed in U.S. Pat. No. 5,483,668,or U.S. Pat. No. 5,577,047 may be used to provide the mobile terminalwith the function of communicating with two base stations at the sametime. Alternatively, CDMA or TDMA/CDMA systems or similar systems may beused in which the mobile terminals can support communications with morethan one base station at the same time.

A second embodiment of the present invention will be described withreference to FIGS. 5 to 7. The second embodiment includes a mobiletelecommunications network 100 in which mobile terminals 114, 119 maycommunicate with more than one base station 101–103, 106–108, 111–113,116–118 simultaneously via radio links on a radio air interface. As anexample, the communication system 100 may use a spread spectrum accessmethod for the mobile terminals 114, 119, e.g. a CDMA, an FDMA/CDMA, aTDMA/CDMA, or an FDMA/TDMA/CDMA system or similar, in particular anysystem using direct sequence spread spectrum techniques. Typically, eachbase station 101–103; 106–108; 111–113; 116–118 will transmit a beaconor pilot signal which can be processed by any mobile terminal. In anexemplary CDMA system, each base station transmits a pilot signal havinga common PN spreading code that is offset in code phase from the pilotsignal of other base stations. During system operation, the mobile unitis provided with a list of code phase offsets corresponding toneighboring base stations surrounding the base station through which acommunication is established. The mobile unit is equipped with asearching element that allows the mobile unit to track the signalstrength of the pilot signal from a group of base stations including theneighboring base stations. The pilot signal will be typically used by amobile terminal for initial synchronisation.

Each group of base stations 101–103; 106–108; 111–113; 116–118 may becontrolled by a site controller 105; 110; 115; 120. Each triplet of basestations 101–103; 106–108; 111–113; 116–118 may serve the three sectorsof one site. Site controllers 105; 110; 115; 120 communicate with a basestation controller (BSC) 125 which in turn may be connected to otherswitches within the network and to other networks such as a pubictelephone network (PSTN). In particular, network 100 may be configuredso that each BSC 125 can communicate with any of its neighbour BSC'sdirectly. The protocols used for communicating between the BSC, the sitecontrollers and other switches in the networks are not considered to bea limitation on the present invention and may be, for example, an IP,and IP/TCP, an ATM protocol or any other suitable protocol. Softerhandover controllers (SHC) 122, 124, each of which is associated withone or more site controllers 105, 110, 115, 120 may be provided forcombining signals from several base stations to improve overallreception.

As shown in FIG. 6 the mobile terminal 114 is communicatingsimultaneously with base stations 102, 103 via two radio links in a softhandover situation. Thus, base stations 102, 103 are in the Active setof mobile terminal 114. When a pilot or beacon signal of a base stationsuch as 106 in the Neighbour Set exceeds a predetermined thresholdlevel, for instance, the base station 106 may be added to the CandidateSet and removed from the Neighbour Set of the mobile terminal 114. Themobile terminal 114 may either communicate a message to the originalbase stations 102, 103 identifying the new base station 106 and adecision made as to whether to establish communication between the newbase station 106 and the mobile terminal 114 or, which is the preferredembodiment of the present invention, the mobile terminal 114 decideswhether communications should also begin with base station 106. Shouldthe communication system 100 or the mobile terminal 114 decide a softhandover is necessary, the mobile terminal 114 sends a handover requestto the new base station 106 with identifying information about themobile terminal 114 as well as explicit information defining each of thenodes of the network which support the current communication. Thehandover request may include the or a currently authenticated signature,a request to establish communications with the target base station 106,a list of the addresses of the nodes of the network which are involvedin the present call and, optionally, a specification of the appropriateQoS of the communication. This list will include at least the addressesof site controller 105, and the BSC 125 and optionally the SCH 105 andbase stations 102, 103 or more nodes higher in the network. On receiptof the message, base station 106 examines the list of addresses of thenodes supporting the current call as well as the signature of mobileterminal 114. As explained with respect to the first embodiment it ispreferred if a target base station such as 106 is able to verify thesignature of mobile terminal 114 without having to interrogate thenetwork. Accordingly, the pre-authenticated signature or list ofsignatures is previously sent by the network to all the nodes supportingthe current communication path and stored in each one. The target basestation 106 determines that the BSC 125 is common to both the list ofaddresses and its own usual communication path through the network, i.e.the BSC 125 is part of its communication tree. Base station 106 theninstructs BSC 125 to set up new path including the new radio linkbetween base station 106 and mobile terminal 114. The command includesthe signature of the mobile terminal 114. The BSC 125 compares thereceived signature with the signature or the list of signatures it haspreviously received from the network. Assuming the BSC 125 finds thesignature, i.e. it has already been authenticated, BSC 125 instructs theSHC 122 to make ports available to carry communications to and from basestation 106 and to soft combine the new communication to and from basestation 106 with the communications from base stations 102, 103. If thisis not possible as not all the communications go through SHC 122 thesoft combining may be done in BSC 125. Once the new path is set up,target base station 106 acknowledges the handover request to mobileterminal 114. The acknowledgement message may identify a new Active Setto the mobile terminal 114 that includes the original base stations 102,103 and the new base station 106. The mobile terminal 114 searches forthe information signal (pilot) transmitted from the new base station 106and communication is established with the new base station 106 withouttermination of communication through the original base stations 102, 103(FIG. 7).

If the target base station 106 does not recognise any address in thelist of addresses from the mobile 114 which is known to be in thecommunication tree of the target base station 106, an alternativeprocedure is necessary. The target base station may pass the handoverrequest and the list of addresses to the BSC 125 which then determinesif one of the addresses relates to a network node which is associatedwith this BSC 125, e.g. a neighbouring BSC. If so, the BSC 125 sets upthe new path to the neighbouring BSC and to the target base station 106using a BSC-BSC direct communication link mentioned above. If BSC 125does not recognise any of the addresses, the handover request may besent up the hierarchical layers of the network until a node is foundwhich can connect to one of the nodes whose address is in the list.Alternatively, if a suitable switch is not found within a ore-determinedtime, a new route may be established to target base station 106 usingconventional techniques.

When the mobile terminal 114 is communicating through two or more basestations 102, 103, 106, it continues to monitor the signal strength ofpilot signals from the base stations of the Active Set, the CandidateSet, and the Neighbour Set. Should the signal strength corresponding toa base station 102, 103, 106 of the Active Set drop below apredetermined threshold for a period of time, or the signal qualitydeteriorate so that it no longer meets the QoS requirements, thecommunication system 100 or more preferably the mobile terminal 114 maydecide to terminate communications through this base station 102, 106.

In accordance with a modification of this embodiment communication froma mobile terminal may be restricted to a limited number of base stationssimultaneously, e.g. two. This reduces the simultaneous use of radio andnetwork resources for the same communication. However, although only twobase stations are used simultaneously, the above procedure guaranteesthat any new communication path set up to a potential target basestation meets the QoS requirements and also includes a re-use of the oldcommunication path.

As described above the procedure in accordance with the presentinvention is described as being used to decide a member of the activelist. In an alternative of the second embodiment, the same procedure maybe used to determine a new member of the Candidate list. In this casethe same procedure as described above is used once a pilot or beaconsignal of a base station in the Neighbour Set exceeds a predeterminedthreshold level. This target base station is not immediately added tothe Candidate list but rather the procedure for setting up the new pathin the network is carried out as described above.

The mobile terminal sends the list of addresses of the nodes of thenetwork supporting the existing call with a request for communication tothe target base station and the new path is set up to the target basestation making reuse of the existing route. Thus, after carrying out theprocedure in accordance with the present invention, parallel routes areset up through the network each capable of meeting the QoS requirementof the current transmission, however, only the old route or routessupports or support the current communication. The final decision tobring a member base station of the Candidate list to the Active list andto start an active duplex communication therewith can be made at a latertime. In accordance with this embodiment the network resources are setup for each member of the candidate list but the radio resources areonly committed at a later time.

This ties up the network resources possibly unnecessarily but networkresources are normally not the limiting resources compared with theradio resources.

In accordance with a third embodiment of the present invention acombination of route extension and partial path replacement duringhandover with optional partial re-routing after handover is used. Inaccordance with this embodiment one node of the network is designated asan anchor node during the handover and remains as one node of thecommunication path both before and after the handover. Typically, onetype of node, for example, a base station controller or a mobileswitching centre will be assigned the duties of the anchor forhandovers. Whichever network element is chosen for this role, it ispreferably if each such element can communicate with each other elementwithin the network. For instance, as shown in FIG. 8, the anchor nodemay be an anchor BSC 125. As long as any handover can be routed throughthis BSC 125 alone, the procedure as described for the second embodimentis carried out. However, when a handover is necessary to a target basestation such as 201 which no longer has the anchor BSC 125 in itscommunication tree, a different procedure is carried out. Let us assumethat the mobile terminal 114 is currently in communication with basestations 107, 108 and has decided to crate a new radio link to basestation 201 (soft handover). The handover request from the mobileterminal 114 to the target base station 201 preferably includes the or asignature of the mobile terminal 114, the list of addresses of thecurrent nodes (107, 108, 1110, 122, 125) of the current communicationstree and the optional definition of QoS requirements for the new link.The target base station 201 consults the list of addresses, recognisesnone of them and transfers the list of addresses and the other items ofthe handover request to the BSC 225. The BSC 225 then examines the listand identifies the address in the list which relates to a neighbouringBSC, namely the anchor BSC 125 and sends a handover request to BSC 125including the signature of the mobile terminal 114. The BSC 125 beingthe anchor BSC recognises the signature as an authentic one and sets upa communication between itself and the anchor BSC 225 along acommunication path which directly connects the two BSC's 125, 225 whilemaintaining the existing communications to base stations 107, 108. Thenew link is then set up with the target base station 201 using theexisting communications path up to the anchor BSC 125 and then a newpath via the BSC 225, SHC 222, site controller 205 to the target basestation 201. Signal combining of all three links may be carried out inthe anchor BSC 125.

As long as some communication passes through one of the base stations101–108 which are the base stations dependent on the communications treefrom anchor BSC 125, this BSC 125 remains as the anchor. However, afterseveral handovers a time will come when no communications pass throughone of the base stations 101–108 anymore. At such a time a partial pathre-routing and optimisation operation can be carried out to simplify thecommunication path, e.g. by eliminating the anchor BSC 125 in thecommunication path by transferring the anchor property of the BSC 125 tothe new BSC 225. In accordance with the present invention this ispreferably done outside a handover operation. This is also preferablytrue for any path re-routing required to upgrade the QoS of a call.

A fourth embodiment of the present invention will be described as amodification of the second and third embodiments. The fourth embodimentrelates to mobile telecommunication systems which use fast powercontrol, for instance CDMA systems or other systems using spreadspectrum tecniques like direct sequence spreading. Fast power control isdescribed in U.S. Pat. No. 5,056,109, for instance. In fast powercontrol, the transmit power of a mobile terminal is controlled from thebase station. Each base station receiver of a cell measures the strengthof the signal received from each mobile terminal. The measured signalstrength is compared to a desired signal strength level for the mobileterminals in the cell. A power control adjustment command is sent toeach mobile terminal with which the base station transceiver is incommunication. In response to the power adjustment command, each mobileunit increases or decreases its transmit power by a predeterminedamount, e.g. 1 dB or makes no change. The power adjustment command istransmitted by the base station transmitter at a relatively high rate,typically of the order of one per millisecond.

In accordance with the fourth embodiment the fast power control isstarted by the target base station before the new path has been set upthrough the network to the target base station. Thus, even before theradio link for traffic to and from the target base station and themobile terminal has been completely set up, the target base stationstarts to fast power control the mobile terminal. Hence, the fourthembodiment is a modification of either the second or third embodiment(other details remaining the same) in which fast power control isstarted by the target base station substantially immediately on receiptof the handover request and before the receipt of the handover commandfrom the mobile terminal 114. Other details of the second and thirdembodiments remain as previously described. The fast power controlcommands may be sent in a suitable channel from the target base stationto the mobile terminal. The fast power control of the mobile terminalbefore the communication from the target base station may deny themobile terminal of the spatial diversity provided by the radio link fromthe target base station whcih may result in power being reduced and theloss or worsening of the reception of the existing radio links. It ispreferred, however, if one mobile terminal receives poor reception for ashort period of until the handover is complete rather than endangeringmany communiactions in the target cell.

While the invention has been shown and described with reference topreferred embodiments, it will be understood by those skilled in the artthat various changes or modifications in form and detail may be madewithout departing from the scope and spirit of this invention as definedin the attached claims. For example, the present invention is notlimited to handover procedures. For instance, if the communication witha current base station is lost the mobile terminal may attempt callre-establishment with the previous base station or an alternative basestation, whichever has the pilot or beacon with the best signal qualityusing a request similar to the forward handover request described withrespect to the above embodiments. The mobile terminal sends the list ofaddresses of the nodes of the network supporting the now-lost call withthe call re-establishment request to the chosen target base station. Asthe network receives exact details of the nodes in the networksupporting the now-lost call via the new base station, the call can bere-established in the minimum of time. As the target base station mayaddress a suitable switch in the existing communication path quickly anddirectly thanks to the explicit address information in the request fromthe mobile terminal there is a significantly improved possibility thatthe call is still pending at the chosen switch.

1. A method of operating a telecommunications system in which mobileterminals may communicate with base stations over an air interface and acommunications network is provided for linking each base station toother points in the network via one or more nodes, a communication toanother user terminal being supported by one or more first radio linksbetween one or more current base stations and a mobile terminal througha plurality of current nodes of the system, the method comprising thesteps of: providing to the mobile terminal information definingexplicitly at least some of the current nodes of the communicationsnetwork supporting the communication; and, in preparation for setting upa further radio link between the mobile terminal and a target basestation while the current one or more first radio links are stillsupporting the communication or one or more of the first radio links hasjust been lost: the mobile terminal transmitting the informationdefining explicitly at least some of the current nodes of thecommunications network supporting the communication to the target basestation.
 2. A method according to claim 1, wherein the information is alist of addresses of the relevant network nodes.
 3. A method accordingto claim 1, further comprising the step of at least partially re-usingthe old communication path supporting the one or more first radio linksfor the new communication path including the further radio link, wherebythe reused part of the old communication path terminates on one of thenodes defined in the information.
 4. A method according to claim 1,wherein the method is part of a handover procedure or callre-establishment or assignment of a target base station to candidate setfrom neighbour set or assignment of a target base station from candidateset to active set.
 5. A method according to claim 4, wherein the methodis part of a handover procedure and the further radio link is set upbefore the one or more first radio links are terminated.
 6. A methodaccording to claim 1, wherein the explicit information defining nodessupporting the existing communication includes explicit informationdefining at least two levels of a hierarchical telecommunicationsnetwork.
 7. A method of operating a telecommunications system in whichmobile terminals may communicate with base stations over an airinterface and a communications network is provided for linking each basestation to other points in the network via one or more nodes, acommunication to another user terminal being supported by one or morefirst radio links between one or more current base stations and a mobileterminal through a plurality of current nodes of the system, the methodcomprising the steps of: providing to the mobile terminal informationdefining explicitly at least some of the current nodes of thecommunications network supporting the communication; and, in preparationfor setting up a further radio link between the mobile terminal and atarget base station while the current one or more first radio links arestill supporting the communication or one or more of the first radiolinks has just been lost: the mobile terminal transmitting theinformation defining explicitly at least some of the current nodes ofthe communications network supporting the communication to the targetbase station, and providing the mobile terminal with pre-authenticatedreference data for that mobile terminal; copying the pre-authenticatedreference data to at least some of the current nodes of thecommunications network supporting the communication; and, in preparationfor setting up the further radio link between the mobile terminal andthe target base station: the mobile terminal transmitting at least apart of the pre-authenticated reference data to the target base station.8. A method of operating a telecommunications system in which mobileterminals may communicate with base stations over an air interface and acommunications network is provided for linking each base station toother points in the network via one or more nodes, a communication toanother user terminal being supported by one or more first radio linksbetween one or more current base stations and a mobile terminal througha plurality of current nodes of the system, the method comprising thesteps of: providing to the mobile terminal information definingexplicitly at least some of the current nodes of the communicationsnetwork supporting the communication; and, in preparation for setting upa further radio link between the mobile terminal and a target basestation while the current one or more first radio links are stillsupporting the communication or one or more of the first radio links hasjust been lost: the mobile terminal transmitting the informationdefining explicitly at least some of the current nodes of thecommunications network supporting the communication to the target basestation, and the target base station beginning fast power control withthe mobile terminal in preparation for setting up the further radio linkbetween the mobile terminal and the target base station before the paththrough the network supporting the further radio link is complete.
 9. Atelecommunication system in which mobile terminals communicate with basestations over radio links, comprising: a communications network forlinking each base station to other points in the network via one or morenodes, a communication between a mobile terminal and another userterminal being connected via one or more first radio links to one ormore current base stations and through a plurality of current nodes ofthe network, wherein, in preparation for setting up a further radio linkbetween the mobile terminal and a target base station while the currentone or more first radio links are still supporting the communication orone or more of the first radio links has just been lost, the mobileterminal is adapted to transmit to the target base station informationdefining explicitly at least some of the current nodes of thecommunications network supporting the communication.
 10. A systemaccording to claim 9, wherein the information is a list of addresses ofthe relevant network nodes.
 11. A system according to claim 9, whereinthe communication path including the further radio link partiallyre-uses the communication path including the one or more first radiolinks, the reused part terminating on one of the nodes defined in theinformation.
 12. A system according to claim 9, wherein the explicitinformation defining nodes supporting the existing communicationincludes explicit information defining at least two levels of ahierarchical telecommunications network.
 13. A telecommunication systemin which mobile terminals communicate with base stations over radiolinks, comprising: a communications network for linking each basestation to other points in the network via one or more nodes, acommunication between a mobile terminal and another user terminal beingconnected via one or more first radio links to one or more current basestations and through a plurality of current nodes of the network,wherein, in preparation for setting up a further radio link between themobile terminal and a target base station while the current one or morefirst radio links are still supporting the communication or one or moreof the first radio links has just been lost, the mobile terminal isadapted to transmit to the target base station information definingexplicitly at least some of the current nodes of the communicationsnetwork supporting the communication and wherein the system is adaptedto provide the mobile terminal with pre-authenticated reference data forthat mobile terminal and for copying the pre-authenticated referencedata to at least some of the current nodes of the communications networksupporting the communication, and, in preparation for setting up thefurther radio link between the mobile terminal and the target basestation, the mobile terminal is adapted to transmit to the target basestation at least a part of the pre-authenticated reference data.
 14. Atelecommunication system in which mobile terminals communicate with basestations over radio links, comprising: a communications network forlinking each base station to other points in the network via one or morenodes, a communication between a mobile terminal and another userterminal being connected via one or more first radio links to one ormore current base stations and through a plurality of current nodes ofthe network, wherein, in preparation for setting up a further radio linkbetween the mobile terminal and a target base station while the currentone or more first radio links are still supporting the communication orone or more of the first radio links has just been lost, the mobileterminal is adapted to transmit to the target base station informationdefining explicitly at least some of the current nodes of thecommunications network supporting the communication and wherein thetarget base station is adapted to begin fast power control with themobile terminal station before the path through the network supportingthe further radio link is complete.
 15. A method of operating atelecommunications system in which mobile terminals may communicate withbase stations over an air interface and a communications network isprovided for linking each base station to other points in the networkvia one or more nodes, a communication to another user terminal beingsupported by one or more first radio links between one or more currentbase stations and a mobile terminal through a plurality of current nodesof the system, the method comprising the steps of: providing the mobileterminal with pre-authenticated reference data for that mobile terminal;copying the pre-authenticated reference data to at least some of thecurrent nodes of the communications network supporting thecommunication; and, in preparation for setting up a further radio linkbetween the mobile terminal and a target base station while the currentone or more first radio links are still supporting the communication orone or more of the first radio links have just been lost: the mobileterminal transmitting at least apart of the pre-authenticated referencedata to the target base station.
 16. A telecommunication system in whichmobile terminals communicate with base stations over radio links,comprising: a communications network for linking each base station toother points in the network via one or more nodes, a communicationbetween a mobile terminal and another user terminal being connected viaone or more first radio links to one or more current base stations andthrough a plurality of current nodes of the network, wherein, the systemis adapted to provide the mobile terminal with pre-authenticatedreference data for that mobile terminal and for copying thepre-authenticated reference data to at least some of the current nodesof the communications network supporting the communication, and, inpreparation for setting up a further radio link between the mobileterminal and a target base station while the current one or more firstradio links are still supporting the communication or one or more of thefirst radio links have just been lost, the mobile terminal is adapted totransmit to the target base station at least a part of thepre-authenticated reference data.
 17. A mobile terminal for use in atelecommunication system in which mobile terminals communicate with basestations over a radio interface via one or more radio links, the mobileterminal being adapted to transmit to a target base station informationdefining explicitly at least some of the current switching nodes of thecommunications network supporting an existing communication between themobile terminal and one or more current base stations over one or morecurrent radio links in preparation for setting up a new radio linkbetween the mobile terminal and the target base station while the one ormore current radio links are supported or have just been lost.
 18. Amobile terminal according to claim 17, wherein the information is a listof addresses of the relevant network nodes.
 19. A mobile terminalaccording to claim 17, wherein the explicit information defining nodessupporting the existing communication includes explicit informationdefining at least two levels of a hierarchical telecommunicationsnetwork.
 20. A network element for use in a telecommunication system inwhich mobile terminals communicate with base stations over radio links,the network element being adapted to receive information explicitlydefining at least some of the current nodes of the communicationsnetwork supporting an existing communication between a mobile terminaland one or more current base stations over one or more current radiolinks, and for setting up a communications path with a further networkelement using the explicit information in preparation for setting up afurther radio link between the mobile terminal and a target base stationwhile the current radio links are still supported or have just beenlost.